A moving picture like a video movie may usually be divided into a plurality of still pictures, each of which is encoded for being digitized in order to be stored in a memory and in order to be transferred. A term “still picture” as mentioned above is hereinafter shortened as “picture”. A picture may be divided into a plurality of blocks each of which is formed by a given number of pixels, i.e. sixteen by sixteen, eight by eight, four by four, etc. A block of sixteen by sixteen pixels is called a macroblock. The picture is encoded through a discrete cosine transform, a quantization process and a Huffmann encoding process on a block by block basis.
A method of encoding a picture using information within that picture only is called intra-encoding. Another method of encoding using a plurality of differentials between a picture to be encoded and a relatively past picture is called inter-encoding. An intra-encoded picture is called an I-picture. An inter-encoded picture is called a P-picture.
A picture encoded on a block by block (often macroblock by macroblock) basis may cause a problem of blocking noise, i.e. an occurrence of image discontinuity on a border between two adjacent blocks after being decoded. A standard named H.264/AVC has thereby adopted a technique of a blocking noise suppressor called a deblocking filter for smoothing and suppressing the blocking noise.
According to the above standard, a plurality of adjacent macroblocks of a picture are combined one another to form a slice while the picture is being encoded by an encoder. The encoder gives the encoded picture an indication if the encoder runs the encoded picture through an encoder's deblocking filter on a slice by slice basis.
A decoder decodes the encoded picture to produce a decoded picture, and runs the decoded picture through a decoder's deblocking filter if the encoder has run the encoded picture through the encoder's deblocking filter. The decoder has the decoded picture bypass the decoder's deblocking filter if the encoder has had the encoded picture bypass the encoder's deblocking filter.
In general, a deblocking filter needs lots of operations of an encoder's processor and of a decoder's processor. A heavily loaded decoder may cause a processing delay and may not decode all pictures to be decoded. Consequently there may be a lack of a frame, a lack of smoothness, etc. in a decoded moving picture.
A decoder is disclosed in Japanese Patent Publication (Kokai), No. 2005-86830, which runs an encoded picture not through an encoder-chosen deblocking filter but through a deblocking filter of a reduced processing load, in a case where the encoder-chosen deblocking filter has a disadvantage from a viewpoint of the processing load.
In a case where the encoded picture is only given an indication if the encoder has run the encoded picture through the encoder's deblocking filter, however, it may be unclear for the decoder how to reduce the processing load of the deblocking filter. In a case where the decoder determines that all the encoded pictures may not be decoded from a viewpoint of the processing load, it may be unclear how to determine which one of the encoded pictures may bypass the deblocking filter.